1. Field
The presently disclosed subject matter relates to a character type vertical alignment mode liquid crystal display (LCD) device.
2. Description of the Related Art
In a prior art character type vertical alignment mode LCD device, since liquid crystal molecules are vertically aligned with respect to substrates while applying no voltage thereto, the black representation is excellent. Also, when an optical compensation plate or a retardation film having a negative optical isomer property is introduced onto one or both polarizers, the viewing angle properties are very excellent (see: JP2005-234254A).
Also, a rubbing aligning process or an ultraviolet ray aligning process is performed upon alignment layers, to thereby realize a mono-domain alignment in a vertical alignment mode liquid crystal layer. On the other hand, slits are provided on electrode layers or ridges are provided on substrates, to thereby realize a multi-domain alignment in a vertical alignment mode liquid crystal layer. Particularly, the above-mentioned mono-domain aligning process can make the alignment state of the vertical alignment mode liquid crystal layer uniform regardless of whether or not a voltage is applied thereto.
Further, in order to avoid alignment defects in the vertical alignment mode liquid crystal layer while applying a voltage thereto, a pretilt angle is allocated so that liquid crystal molecules in the vertical alignment mode liquid crystal layer are tilted a little from a vertical angle (90°) with respect to the substrates while applying no voltage thereto.
In the above-described prior art character type vertical alignment mode LCD device without requiring thin film transistors (TFTs), a multiplexing driving is used. A typical multiplexing driving is based on an optimal bias method whose driving waveforms are an in-frame-reversal driving waveform or a line-reversal driving waveform (hereinafter, referred to as an A-waveform), a frame-reversal driving waveform (hereinafter, referred to as a B-waveform), and a multi-line-reversal driving waveform (hereinafter, referred to as a C-waveform). Note that the B-waveform is now often used in view of the small power consumption.
In the above-described prior art character type vertical alignment mode LCD device, however, since the anchoring force of the direction of the azimuth of liquid crystal on the plane of the substrates is weaker than that of a horizontal alignment mode LCD device such as a twisted nematic-mode (TN-mode) LCD device, when the direction of the azimuth of liquid crystal on the plane of the substrates is deviated by some external factors from a direction set by an alignment process, the retardation would be partly changed, so that a low transmittivity region would be visible as a “black shadow region” within a white pixel (dot) of the vertical alignment mode liquid crystal layer while applying a voltage thereto. Also, if the viewing angle is changed, the black shadow region would be visible as a “rough region”. Further, if one black shadow region within one white dot reaches another black shadow region of its adjacent white dot, a plurality of black shadow regions are visible as an “irregularly-continuous region” within continuous white dots. The phenomenon of such a black shadow region, a rough region and an irregularly-continuous region is called a dynamic misalignment (DMA) phenomenon which would not only decrease the uniformity of representation of dots, but would erase patterns represented by dots.
The generation state of the above-mentioned DMA phenomenon may be changed due to various internal factors such as a pretilt angle affecting the anchoring force of the azimuth of the direction of liquid crystal on the plane of the substrates and the frame response phenomenon of liquid crystal.
Also, the generation state of the above-mentioned DMA phenomenon may be changed due to some external factors. One of the external factors is an oblique electric field generated between electrode layers, i.e., a segment electrode layer and a common electrode layer. In more detail, an oblique electric field is generated between an edge of one segment electrode of the segment electrode layer and an even portion of one common electrode of the common electrode layer. Similarly, an oblique electric field is generated between an edge of one common electrode of the common electrode layer and an even portion of one segment electrode of the segment electrode layer. Particularly, the generation state of the DMA phenomenon in the vertical alignment mode LCD device is strongly affected by the above-mentioned oblique electric field. That is, since the liquid crystal in the vertical alignment mode LCD device is of a negative type, the director of liquid crystal can easily fall along a direction perpendicular to an electric line of force of an electric field applied thereto, so that the director of liquid crystal easily falls along a direction perpendicular to an electric line of force of the above-mentioned fringe field. Therefore, if the director of liquid crystal is different from a director of liquid crystal set by an alignment process, a black shadow region would be visible between the boundaries of the segment and common electrodes.
In the above-described prior art character type vertical alignment mode LCD device, since the pretilt angle is around 90° so that the anchoring force of liquid crystal along the direction of the azimuth thereof on the plane of the substrates is very small, and also, the liquid crystal is in a high response speed state, the liquid crystal is easily moved along the direction of the azimuth thereof on the plane of the substrates. That is, the above-mentioned high pretilt angle is required to improve the sharpness for high viewing angle properties at a high duty ratio driving operation. Also, the above-mentioned high response speed state can be realized by the low viscosity of liquid crystal, a thin thickness of a liquid crystal layer, a high operational temperature and so on. As a result, a director of liquid crystal would be generated from a start position where an oblique electric field whose direction is different from the direction of the azimuth of liquid crystal set by the alignment process is generated along a direction different from the direction of azimuth of liquid crystal set by an alignment process. In this case, since liquid crystal molecules have forces to make them parallel with each other, and the anchoring force of liquid crystal along the direction of the azimuth thereof on the plane of the substrates is very small, as stated above, a black shadow region with deviated directors of liquid crystal is spread gradually from the above-mentioned start position to its peripheral positions. Thus, a large number of directors of liquid crystal are deviated from the alignment direction set by the alignment process.
In order to avoid the generation of the above-mentioned black shadow region, one approach is to suppress the frame response phenomenon. That is, a high frequency driving method increasing the frame frequency and using the A-waveform, the C-waveform or a multi-line addressing (MLA) waveform is carried out to decrease a pulse interval by a multiplexing driving. However, this high frequency driving method would increase the power consumption and also, would increase the crosstalk phenomenon by resistance components of the electrode layers.